...one of the most highly
regarded and expertly designed C++ library projects in the
world.

— Herb Sutter and Andrei
Alexandrescu, C++
Coding Standards

There are cases, especially in the domain of numeric computation, when one wants to perform some part of the calculations at compile-time, and then pass the results to a run-time part of the program for further processing. For example, suppose one has implemented a complex compile-time algorithm that works with fixed-point arithmetic:

// fixed-point algorithm input typedef mpl::vector< mpl::fixed_c<-1,2345678> , mpl::fixed_c<9,0001> // .. , mpl::fixed_c<3,14159> > input_data; /* complex compile-time algorithm */ typedef /*...*/ result_data;

Suppose the `result_data` here is a sequence of `mpl::fixed_c` types that keeps the results of the algorithm, and now one wishes to feed that result to the run-time part of the algorithm. With MPL she can do this:

double my_algorithm() { // passing the results to the run-time part of the program std::vector<double> results; results.reserve(mpl::size<result_data>::value); mpl::for_each<numbers,_>( boost::bind(&std::vector<double>::push_back, &results, _1) ); // ... }

The `for_each<numbers,_>(...)` call is what actually transfers the compile-time `result_data` into run-time `results`. `for_each` is a function template declared as:

template< typename Seq , typename TransformOp , typename F > void for_each(F f) { // ... }

To call the function, one is required to explicitly provide two actual template parameters, a compile-time sequence `Seq` and a unary transformation metafunction `TransformOp`, plus a run-time function argument `f` (in our example, `numbers`, `_`, and `boost::bind(...)` correspondingly). `f` is a function object which `operator()` is called for every element in the `Seq` tranfromed by `TransformOp`.

Applying this to our example, the

mpl::for_each<numbers,_>( boost::bind(&std::vector<double>::push_back, &results, _1) );

call is roughly equivalent to this:

f(mpl::apply< _,mpl::at_c<result_data,0>::type >::type()); f(mpl::apply< _,mpl::at_c<result_data,1>::type >::type()); // ... f(mpl::apply< _,mpl::at_c<result_data,n>::type >::type());

where `n == mpl::size<result_data>::type::value`.